SAN FRANCISCO — The San Francisco Public Utilities Commission is pursuing a plan to store water underground that can be pumped out in time to supply customers in a drought, given the uncertainty of California's water future.

Officials say the natural groundwater aquifer that sits under north San Mateo County will someday be full enough to send 7.2 million gallons per day to SFPUC customers in San Francisco, San Mateo and Alameda counties and much of Santa Clara County for a period of seven and a half years, longer than the last historic drought period in California.

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Global warming, and the resulting anticipated loss of Sierra snowpack that feeds the Hetch Hetchy reservoir, have played a part in the SFPUC's long-term planning for water security here in the Bay Area, said Ellen Levin, deputy manager of San Francisco's regional water system.